Welding system



March 18, 1941. R, H. WRIGHT WELDING SYSTEM Filed March 23, 1939 3 Sheets-Sh'e'nt 1 WITNESSES;

March 18, 1941. R. H. WRIGHT WELDING SYSTEM 3 shoets- -sheet- 2 Filed March 23, 1.939

INVENTOR Aa/p/z hf wrg /zzz WITNESS s:

March 18, 1941. R. H. WRIGHT WELDING SYSTEM 3 Sheets-Sheet 3 Filed March 2.3, 1939 QNN WITNESSES: 'g

Patented Mar. 18, 1941 UNITED STATES PATENT OFFICE Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation oi! Pennsylvania Application March 23,

13 Claims.

My invention relates, generally, to welding systems, and more particularly, to direct-current resistance welding systems for welding pipe, conduit and the like.

In the manufacture of pipe and conduit by the resistance welding method, currents of from 40,000 to 200,000 amperes are normally required at a relatively low voltage of from 2 to '7 volts across the electrode members, depending on the size or the pipe and the thickness and nature of the material used. In order to supply direct currents of such magnitude, generators or the homopolar or unipolar type have heretofore been used to some extent, since generators of this type may be built having suilicient capacity to supply the required welding current from a single generator. However, the highly specialized construction of this type of generator makes it relatively expensive to build and maintain. Because of the tremendously strong field required by a generator of this type, it is not possible to maintain accurate control of the welding current by control of the field excitation, the inertia of the field system being so great that, in some 25 instances, it requires from three to four minutes for the field to build up after the field winding has been energized.

Direct-current generators of the commutator type, while adaptable to accurate control or the current output through control of the field excitation thereof, because of the substantially lower inductance of the field circuit, have not as yet been built in sizes having suflicient capacity to supply the entire welding current required in resistance pipe welding operations from a single generator. In order to supply the necessary current capacity, it, therefore, becomes necessary to operatea plurality of such generators in parallelcircuit relation. Inasmuch as the voltage required across'the welding electrodes for the resistance'welding of pipe is only of the order of from 2 to 7 ,volts, it will be apparent that where a number of direct-current generators are connected in parallel for supplying the necessary welding current, even normal variations in the I 1939, Serial No. 263,751 (01. 219-12) vide direct-current generators of even 25,000 amperes capacity with series field windings, on account of the physical impossibility of providing such windings for carrying currents of this magnitude, it is not, therefore, possible to use equal- 5 izer bus connections for balancing the division of load between a plurality of such generators, so as to overcome the efiects of variations in the terminal voltage of the diiierent machines, resulting from such normal changes in their cir- 10 cult characteristics and permit operation thereof in parallel.

It is, therefore, an object of my invention to provide for controlling a plurality of direct-current generators of the commutator type that are 15 connected for parallel-circuit operation in a resistance welding system so as to maintain a predetermined division of the Welding current among the generators.

More specifically, it is an object of my invention to provide a resistance welding system for pipe welding, utilizing a plurality of direct-current generators of the commutator type connected in parallel-circuit relation to supply welding current to the electrode members, and control the excitation of each generator automatically to maintain a predetermined division of the welding current among the generators without the use of series field windings or equalizer bus connections between the generators. 0

Another object of my invention is to provide in a resistance welding system for controlling, both individually and collectively, a plurality of direct-current generators of the commutator type which are connected in parallel-circuit relation to the electrode members, so as to maintain a predetermined division of the welding current among the generators and maintain a predetermined flow of welding current between the electrode members during a welding operation.

A further object of my invention is to provide for maintaining a predetermined division or welding current among a plurality of relatively low-voltage direct-current generators of the commutator type which are connected for operation in parallel-circuit relation to the electrode members of resistance pipe welding'apparatus, and control the excitation of the generators so as to provide a quick response direct-current source for maintaining a predetermined flow of welding -current between the electrode members during a welding operation and eflect energization of the electrode members in accordance with the movement of the pipe member toward and away irom the electrode members.

Other objects will, in part, be obvious, and will, in part, appear hereinafter.

For a complete understanding of the nature and scope of my invention, reference may be had to the following detailed description taken in connection with the accompany-ing drawings, in which: I

Figure 1 is a diagrammatic view of a resistance pipe welding system embodying the principal features of a preferred form of my invention;

Fig. 2 is a diagrammatic view of a resistance pipe welding system embodying an alternate form of my invention; and

Fig. 3 illustrates diagrammatically the application of another embodiment of the invention to a resistance pipe welding system.

In practicing a preferred form of my invention, a plurality of direct-current generators of the commutator type are connected for operation in parallel-circuit relation to supply welding current to the electrode members of resistance pipe welding apparatus which engage a pipe member on opposite sides of the seam that is to be welded. A predetermined division of the welding current among the generators is maintained preferably,

by individually controlling the excitation of each of the generators in accordance with the differential between the welding current of the individual generator and, either the welding current supplied by another of the generators or the total welding current supplied to the electrode members. The excitation of the generators may furthermore be controlled collectively, so as to maintain a predetermined flow of welding current during the welding operation, and the energization of the electrode members is controlled so as to prevent any burning thereof as the pipe member engages or leaves the electrode members.

Referring particularly to Figure 1, the reference numerals IO' and I2 denote direct-current generators of the commutator type which are connected in parallel-circuit relation to the welding busses l6 and M3 for supplying welding current to the rotatable electrode members 20 and 22, which are positioned for engaging the pipe member 24 on opposite sides of the seam 26 which is to be welded. Movement of the pipe member 24 relativeto the electrode members 20 and 22 may be effected in any manner well known in the art of pipe welding, such as, for example, by means of separate drive rolls 21, or by driving the electrode members themselves in any suitable manner.

The generators l and I 2 may be provided with suitable driving means, such, for example, as a motor 28 which is connected to a suitable source of power by conductors 29.

The generators l0 and I2 are provided with field windings 30 and 32, respectively, which are connected through a control switch 34 to the control busses 36 and 38, which may be energized from any suitable substantially constant potential source, such as the auxiliary generator 40. A field discharge resistor 4| is provided for connection across the field windings 30 and 32 upon deenergization thereof to prevent dangerously high open circuit voltages from being induced therein.

In order to maintain a predetermined division of the welding current between the generators Ill and I2 during a welding operation, a balance resistor 42 may be connected across the field windings 30 and 32 with an adjustable contact arm 44 thereof connected to a common point 45 midway between the field windings. The contact arm 44 may be actuated by a motor 48 having 0pprising,

positely connected field windings 48 and 50 controlled by a relay 52, so as to actuate the contact arm 44 of the balance resistor to vary the relative amount of resistance shunting the two field windings and thus vary excitation of the field windings 30 and 32 relative to each other, preferably in accordance with the differential between the welding current supplied by each generator.

As illustrated, the relay 52 may be in the form of a balance relay having a pivoted contact arm 54 operable to engage either of the stationary contact members 56 or 58, dependent on the relative energization of the opposed operating windings 60v and 62 thereof. The operating windings 60 and 62 are energized in accordance with the welding current supplied by the generators Ill and I2, being, for example, connected to shunt members 66 and 68, in the load circuits of the generators Ill and I2, respectively.

In order to secure the desired division of welding current between the generators l0 and I2, the relay 52 is connected so that upon a decrease in the welding current'supplied by the generator I0, the pull exerted by the operating winding 62 actuates the contact arm 54 of the relay to engage the stationary contact member 58 and effect energization of the field winding 50 and armature of the motor 46 to actuate the movable contact arm 44 of the balance resistor 42 in direction to increase the energization of the field winding 30 of generator l0 anddecreasing the energization of the field winding 32 of generator l2, so as to correct any unbalance existing between the welding current supplied by the two generators. It is to be understood that the generators l0 and I2 may carry equal loads or they may divide the total welding current load in any other desired proportion.

In order to maintain a predetermined fiow of total welding current during a welding operation, regulating means may be provided, comfor example, an adjustable rheostat 12, which is connected in series circuit relation with the field windings 30 and 32 of the generators when the control switch 34 is closed, for collectively controlling the energization thereof. A motor 14, having oppositely connected field windings I6 and I8, the energization of which is controlled by a relay 80, may be utilized for actuating the movable contact arm of the rheostat 12.

The relay 8!] may, for example, comprise a pivoted contact arm 82 which is actuated, through energization of an operating winding 84, against the biasing force of a spring 86 to selectively engage the stationary contact members 88 and 90, depending upon whether the potential between the welding busses l6 and I8 is less than or greater than a predetermined value. A variable resistance 92 may be connected in the energizing circuit of the operating winding 84 of the relay 80 for adjusting the relay to maintain different predetermined values ofwelding current in the welding circuit during the welding operation.

In order to automatically control the energizatlon of the electrode members and 22 so as to prevent the engagement thereof by the pipe member 24 while the electrode members are energized, or the separation therefrom of the pipe member while they are energized, auxiliary switches 94 and 96 may be utilized. These .switches may be positioned adjacent the electrode members on opposite sides thereof as shown so as to be actuated by engagement with the pipe member '24 in its passage between the electrode members. By connecting the auxiliary switches 84 and 88 in series-circuit relation, with the operating coil 88 of the control relay 34, the energization of the field windings 30 and 32 may be effected in accordance with the position of the pipe member 24 relative to the electrode members. whereby the electrode members are energized immediately following the engagement of the pipe member 24 therewith and deenergized immediately prior to its disengagement therefrom.

In order to reduce the time required for the welding generators I0 and I2 to build up their voltage to the desired normal value provision is made for forcing the excitation of their field windings for a predetermined time after the welding operation is commenced. This may be accomplished by the use of afield forcing resistor I00 which is connected in series-circuit relation with the field windings 30 'and 32 of the generators and controlled by afield forcing relay I02, the normally-closed contact members I02a thereof being connected to shunt the resistor when the relay is deenergized as shown. The operating winding I04 of the field forcing relay is connected for energization through contact members of the auxiliary switches 84 and 88, so as to be energized upon closure of both of the auxiliary switches. Time delay means, such as the dash pot I08 may be provided for preventing operation of the relay I02 to insert the resistor I00 in the circuit of thefield windings 30 and 32 for a predetermined interval of time after actuation of the auxiliary switches 84 and '98, so as to provide for initially increased excitation of the field windings 30 and 32 for a predetermined time. As soon as the relay I02 opens its contact members mm the resistor I00 is rendered effective to reduce the field excitation of the generators to the normal value as determined by the regulating means 10.

In order to facilitate welding operations when welding relatively short lengths of pipe, an auxiliary relay I08, having an operating winding N0 the energization of which is dependent on the closure of the auxiliary switches 84 and 88, is provided for rendering the regulating means I0 ineffective as soon as the pipe member 24 disengages the auxiliary switch 04, so that the regulating means I0 will not, upon the completion of the passage of a pipe member between the electrode members 20 and 22, continue to attempt to maintain the voltage across-the welding bus members I8 and I8 at the closed circuit value maintained during the welding operation.

In order to render the use of the field forcing relay I02 more effective, it is preferable to momentarily retard the operation of the regulating means as a welding operation is commenced, and for this purpose, the auxiliary relay I08 may be provided with time delay means such as the dash pot I I2 which functions to retard the closing operation of the relay for a predetermined length of time after energization of the electrode members through closure of the auxiliary switches 84 and 88.

The sequence of operations of the system during a welding operation is as follows:

Upon the engagement of the pipe member 24 with the auxiliary switches 84 and 80, contact members 84aand 98a respectively, are closed, establishing energizing circuits for the operating winding 88 of the control relay 34, the operating winding I04 of the field forcing relay I02, and the operating winding IIO of the auxiliary relay I08 which extend from the control bus 38,-

through conductor 3, contact members 88a. and 84a, conductor H4 and operating windings 88, I04 andv H0, in parallel-circuit relation to the bus 38.

Operation of the control relay 34 connects the field windings 30 and 32 of the generators I0 and I 2, respectively, to the control busses 38 and 38 through a circuit extending from bus 38, through the rheostat I2, contact members 34b, field winding 30, field winding 32, contact members 34c and the contact members I02a of the field forcing relay I02, to bus 38. This causes the generators I0 and I2 to develop voltage and the electrode members 20 and 22 are thereby energized to efiect the desired fiow of welding current for welding the pipe seam 28. A predetermined interval of time after the energization of the control relay 34, the field forcing relay I02 operates, opening contact members I02a and inserting the resistor I00 in the field circuit of the generators, thus reducing the energization of their field windings 30 and 32 to a normal value.

The operating coil IIO of the auxiliary relay I08 being also energized, this relay closes after a predetermined time to connect the regulating means 10 to the relay which functions to control the regulating means I0 in accordance with the voltage of the welding busses IB and I8. Should the voltage across the welding busses I6 and I8 rise above a predetermined value, for example, the operating winding 84 actuates the movable contact arm 82 to engage the stationary contact member 90, thus connecting the armature and field winding I8 of the motor 74 across the busses 38 and 38 to effect actuation of the contact arm I5 of the rheostat 12 in a counterclockwise direction to reduce the energization of the. field windings 30 and 32 of the generators and thereby reduce the total welding current to the desired value. Upon a decrease in the voltage across the welding busses I6 and I8, the moving contact arm 82 is actuated in the opposite direction to effect energization of the armature and field winding 18 of the motor 14, and thus increase the excitation of the field windings 30 and 32 so as to increase the welding current the necessary amount.

Should the terminal voltage of the generator I0 vary, relative to the terminal voltage of the generator I2, through any variation in the internal resistance of the armature, the contact resistance of the brushes, or the brush drop, so that the generators I0 and I2 do not divide the total welding current load in the desired proportion, the energization of the operating windings 60 and 62 of the relay 52 varies accordingly. The movable contact arm 54, as a result, is actuated to engage either the stationary contact member 58 or the stationary contact member 58, thus effecting energization of the armature 'of motor 48 and either the field winding 48 or the field winding 50 from the control busses 36 and 38, so as to actuate the movable contact arm 44 of resistor 42 in either a counterclockwise or a clockwise direction to respectively increase or decrease the energization of the field winding 30 relative to that of the field winding 32, depending on whether the welding current supplied by the generator I0 decreases or increases relative to the welding current supplied by the generator I2. In this manner, the relative energization of the field windings 30 and 32 may be suitably varied in accordance with the existence of any differential between the Welding currents supplied by the two generators, so as to immediatelycorrect such unbalance and maintain a predetermined division of the welding current between the generators.

As soon as the pipe member 24 disengages the auxiliary switch 94, the contact members 9411 open. The control relay 34 is thereby deenergized, disconnecting the fieldwindings 30 and 32 from the busses 36 and 38, connecting them across the field discharge resistor 4I through contact members 340. The electrode members 20 and 22 are thereby deenergized in a simple and effective manner, so that there will be no burning of the electrode members or the pipe member through the drawing of any arc as the pipe member leaves the electrode members.

At the same time, the operating winding III) of the auxiliary relay I06 is deenergized, so that the regulating means 10 is rendered inefiective and the operating winding I04 of the field forcing relay I02 is also deenergized, so that the contact members I02a close, again shunting the resistor I to condition the field circuit of the generators for a subsequent energization.

By rendering the regulating means ineffective upon the termination of the welding operation, any tendency of the regulating means to maintain the voltage of the welding busses I6 and I8 at the closed circuit welding value is prevented. The setting of the rheostat I2 is, therefore, retained at a value closely approximating that necessary to provide the value of welding current required-during welding, thus conditioning field circuit of the generators for eifecting a rapid establishment of the desired flow of weld ing current upon the entry of another pipe member between the electrode members and 22.

Referring particularly to Fig. 2 of the drawings, which illustrates an alternate form of the invention, the reference numerals I0 and I2, as hereinbefore, denote generally direct-current generators of the commutator type which are connected in parallel-circuit relation to the welding busses I6 and I8, respectively, for supplying welding current to' the electrode members 20 and 22, which by means ofa circuit breaker I20, which is in this instance, actuated by fluid pressure means I22 under the control of a solenoid operated fluid pressure valve I24, against the force of a spring I26, which normally biases the circuit breaker to the open position. The generators may be driven in any suitable manner, such, for example, as shown in Fig. 1.

In this embodiment of the invention, the field windings 30 and 32 of the generators I0 and I2, respectively, are connected for energization to thecontrol busses 36 and 38, and control exciters I30 and I32 are, respectively, connected in series relation therewith for individually varying the energization of v the respective field windings from the control busses in accordance with variations in the division of the welding current between the generators. The control exciters I30 and I32 are preferably provided with differentially connected field windings I34 and I36, and I38 and I40, respectively.

As will be readily understood the control exciters I30 and I32 may be operated in any one of three different ways. They may be operated in such manner as to develop zero voltage when their associated welding generators are dividing the total load in a predetermined desired manner in which event they would function to either buck or boost the voltage of the control busses 36 and 38 depending upon the load variation of their associated generators.

The control exciters may be also operated as bucking generators or as booster generators. When they are operated as bucking generators they function to buck the voltage of the control busses 36 and 38 at all times in varying degrees dependent upon the load variation of their associated generators andwhen they are. operated as booster generators they function in the opposite manner toboost the voltage of the control busses 36 and 38 in varying degrees dependent upon the load variation of their associated generators. In this instance, however, it will be assumed that the control exciters function in the mannerfirst described wherein they develop zero voltage when their associated generators are driving the total welding load in a predetermined desired manner.

Pilot exciters I42 and I44 are provided for con trolling the excitation of the field windings of the control exciters, in accordance with the load currents of .the generators, having field windings I46 and I48, which are energized in accordance with th welding current supplied by the generators E 0 and I2, respectively. The field wind ings I4 and I48 of the pilot exciters may be energized in any suitable manner. In this in stance they are connected to the armatures I50 and I52 of dynamo-electric machines I54 and I56, the field members I58 and I59 of which comprise substantially U-shaped frame members which are looped about the portions I60 and I6I of the main busses of the generators I0 and I2, so that magnetic fluxes are induced therein pro; portional to the values of the welding currents supplied by the generators. By connecting the armatures I50 and I52 of the dynamo-electric machines I54 and I56 to suitable driving means (not shown) so as to drive them at a substantially constant speed, the field windings I46 and I48 of the pilot exciters I42 and I44 may be energized in accordance with the current supplied by the generators I0 and I2, respectively.

The difierential field winding I34 of the control exciter I30 and the-cumulative field winding I40 *of the control exciter I32 are connected to be energized from the pilot exciter I42, while the difierential field winding I38 of the control exciter I32 and the cumulative field winding I36 of the control exciter I30 are connected for energization from the pilot exciter I44. Switches I62 and I63 may be provided for disconnecting the field'wlnding-s 30 and 32 of the generators I0 and I2 from the control busses when desired, and connecting the field discharge resistors I64 and I85 across the field windings to prevent the building up of high open circuit voltages in the field windings upon disconnection from the control busses.

The auxiliary switches 94 and 96 are, a in the welding system shown in Fig, 1, utilized for controlling the energization of the electrode members 20 and 22 in accordance with the movement of the pipe member 24 relativeto the electrode members. The energizatlon of the operating winding I66 of the solenoid-operated valve I24 may be controlled by, the auxiliary switches 34 and 36 to effect closure thereof only when both switches are closed, and effect the opening thereof as soon as the switch 34 opens.

To control the flow of welding cprrent during a welding operation, regulating means I0 may be provided for controlling the voltage across the control busses 36 and 38. As illustrated, a tapped resistance I72 is connected in series circuit relation with the field winding I14 of the aasasoa generator 40 which energizes the control busses and I0, the winding I'll being, for example, connected for energization to any suitable source such as the generator I'll A plurality of relatively thin resilient contact members I10 are assembled in normally spaced relation by insulating means I10 adjacent to one end. These ends of the contact members IIi are respectively connected to difierent taps of the resistance I12, while the other ends are supported in normally spaced relation by an insulating support member I80. Control means, comprising a pivoted operating arm I82 biased by a spring I" to normally engage and fiex the contact members I", is provided for forcing the normally spaced ends of contact members I18 together, so as to shunt out the tapped sections of the resistor I12 which are connected therebetween. By connecting the operating winding I" of the control means across the welding busses I0 and I 2 in series circuit relation with an adjustable resistance ill. the operating winding I86 may be energised in accordance with the voltage existing across the welding busses I8 and I8 to eifect actuation of the operating arm I82 against the force of the spring its so as to effect separation or the lea! members I10 and insert sections of the resistance I12 into the field circuit of the generator 40, thus controlling the energization of the field windings 3! and 32, so as to maintain diiferent predetermined values of welding current during a welding operation. An auxiliary relay I02 is provided for rendering the regulating means ineifective between successive welding operations and a field the other,

forcing relay I02 is provided for eifecting an increase in the excitation of the field windings 20 and 32 upon the commencement of a welding operation.

By providing the control exciters I30 and I32 of the generators I0 and I2, respectively, with the difierential and cumulative field windings connected in the manner hereinbefore described. it will be understood that upon any decrease or increase in the load of one generator relative to the excitation of the field winding of the pilot exciter associated with that generator will accordingly be varied so as to eifect changes in the excitation of the control exciters of the generators I0 and I2, to immediately compensate for any such change in the division of the welding current and reestablish the balance of the load between the generators.

The operating sequence of the welding system illustrated in Fig. 2 is similar to that of the welding system of Fig. l, which was hereinbeioredescribed in detail. The energisation of the electrode members 20 and 22 is controlled in accordance with the passage of the pipe member 24 relative to the electrode members by the auxiliary switches 04 and 30 controlling the operation of the circuit breaker I23. The operations of the field forcing relay I32, the auxiliary relay I30 and the regulating means 10 are substantially the same as described in connection with Fig. 1.

When any unbalance occurs between the welding current supplied by the generators I3 and I2, the excitation of the field windings I" and I or the pilot exciters is afiected' accordingly through the corresponding change in the excitation of the dynamo electric machines I and Ill. The energization of the cumulative and differential windings of the control exciters I30 and I22 is therebyvaried so as to regulate the relative energization of the field windings 33 and 32 of the generatorsto correct the unbalance. For

example, suppose that because of variations in the armature resistance, contact resistance, or brush drop, the output of the generator II should decrease: The voltage generated by the dynamo electric machine I" would likewise decrease because oi the reduced current in the generator bus portion I00. The energization of the difierential field winding I34 of the control exciter I30 and the cumulative field winding I00 of the control exciter I32 is accordingly decreased.

Under the assumed condition of operation of the control exciters this causes the control exciter I30 to develop a voltage which is cumulative with respect to the voltage of the control busses 36 and 33 and thereby increases the excitation of the generator I0 and the control exciter I22 to develop a voltage which is differential with respect to the voltage of the control busses 30 and 30 to decrease the excitation of the other generator I2, which has assumed more than its share of the total load.

Under these same conditions the total welding load remaining substantially constant, the pilot exciter I, associated with the generator I2, increases its voltage due to the excess load assumed by generator I2 and increases the energization of the differential field winding I38 of the control exclter I32 of generator I2 which further-increases the differential voltage developed by the exciter I32. Since the cumulative field winding of the control exciter I30 is connected to the pilot exciter I its energization islikewise increased which further increases the cumulative voltage developed by the control exciter I30.

Thus it will be apparent that the terminal voltage or generator I0 is increased not only as the result of its reduced load but also as the result of the increased load on the other generator I2. Likewise the terminal voltage of the generator I2 is reduced in accordance with its increased load and the decreased load of generator I0. This results in the restoration oi the desired load division or balance between the generators.

, Referring particularly to Fig. 3, the reference numerals I0, I2 and I 4 denote, respectively, generators of the commutator type which are connected for operation in parallel-circuit relation to energize the welding busses I3 and I3 which supply welding current to the electrode members 20 and 22, and are provided with field windings 33, 32 and 33, respectively, The generators I3, I2 and It may be driven in any suitable manner, such, for example, as shown in Fig. l. Inasmuch as the load and control circuits, and the operation of the three generators is the samefthe following description is limited to the generator II, and circuits and operation of the other generators may readily be understood therefrom.

The field winding 30 of the generator may be connected for energization across the control busses 30 and 38 which are energized from the source 40. In order to control the excitation of the field winding 30 so that the generator I0 will supply a predetermined portion of the total welding current, a rheostat H0 is connected in series circuit relation with the field winding 30, and the energisation of armature and the oppositely connected field windings 2i 2 and 2 It of the operating motor 2" thereof controlled by means of the relay 32. v

As explained in connection with Fig. l, the relay 32 may comprise a pivoted contact arm 04 having oppositely acting operating windings 00 and 32 for urging the contact arm to engage the stationary contact members." and ll,'respective-' of the generator.

1y. The operating winding 62 isenergized in accordance with the welding-current supplied by the generator 10, being, for example, connected across a shunt member I81 in the main circuit The operating winding 60 is energized in accordance with the total welding current, and may be connected to a shunt member in the main welding circuit, or, as illustrated, across the armature 2I8 of a. dynamo-electric machine 220, having a field structure 222 which comprises a substantially U-shaped member disposed in looped relation about the main welding bus in the manner hereinbefore described in connection with Fig. 2. By thus connecting the opposed operating windings 60 and 62 of the relay 52 to be energized respectively in accordance with the total welding current and with the welding current supplied by the generator I, energization of the field winding 30 of the generator It! may be controlled so as to maintain a predetermined relation between these two values of welding current. By similarly controlling the excitation of the generators-l2 and M, a predetermined division of the total welding current may be maintained among the generators.

The total welding current may be maintained at any predetermined suitable value by regulating means 10, which functions to control the energization of the control busses 36 and 38 to which the field windings of all the generators are connected. The operating coil 226 of the regulating means is connected across the armature 2|8 of the dynamo-electric machine 220 for energization in accordance with the total welding current, so as to control the energization of the oppositely connected field windings l6 and 18 of the rheostatactuating motor 14 to vary the energization of the control bus members 36 and 38 from the source '40, as hereinbefore described in detail in connection with Fig. 1.

The energization of the electrode members 2|! and 22 may, as hereinbefore described in connection with the welding system illustrated in Fig. 1, be controlled by controllin the energization of the field windings of the generators through control relays 34, which also control the connection of the field discharge resistor 4| across the field windings upon disconnection from the source 40. Likewise, the field forcing relay I02 and the auxiliary relay I08 function in the same manner set forth in connection with the system of Fig. 1 of the drawings. a 1

From the above-detailed description, taken in connection with the accompanying drawings, it will be understood that I have provided for con trolling the division of the Welding current between a plurality of direct-current generators of the commutator type which are connected for operation in parallel-circuit relation to supply welding current for a pipe welding operation at a relatively low voltage of the order of from 2 to 7 volts across the electrode members.

By utilizing the embodiments of my invention hereinbefore described and shown, it is possible to utilize direct-current generators of the commutator type for supplying direct current of large enough values for the resistance welding of pipe, and accurately control the division of the welding current among the generators without requiring the use of equalizer bus connections, which cannot be used with series windings of any sort on the generators or generators of the capacity required in pip welding applications because of the enormously large conductors necessary to carry the required welding currents.

By controlling the field excitation of the generators collectively and individually in the manner hereinbefore described, it is possible to utilize the quick-response field characteristics of generators of the commutator type, and thus provide a resistance pipe welding system for maintaining accurate control of the welding current without requiring the use of highly specialized and expensive control apparatus.

Since certain changes may be made in the above description, and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all the matter contained in the above description and shown'in the accompanying drawings shall be considered as illustrative, and not in a limiting sense.

I claim as my invention:

1. A power system comprising, a plurality of generators of relatively low terminal voltage connected in parallel circuit relation for supplying current to a work circuit, means for automatically controlling the excitation of each generator to maintain a predetermined division of the load current among the generators, and additional means for controlling the excitation of the generators independently of the aforesaid means to maintain a predetermined flow of load current.

2. The combination in a power system for supplying electrical energy to a work circuit, of a plurality of low voltage direct-current generators connected in parallel-circuit relation for supplying current to the work circuit, means for difl'erentially controlling the excitation of each generator to maintain a predetermined division of the load current among the generators, and

means for continuously controlling the excitation of the generators under load conditions to maintain a predetermined load current. v

3. The combination in a. power system for energizing a load circuit, of a plurality of relatively low-voltage direct-current generators connected in parallel-circuit relation for energizing the load circuit, regulating means operable to control the excitation of the generatorscollectively to regulate-the flow of load current, independently operable means for differentially controlling the excitation of each generator so as to divide the load current between the generators according to a predetermined ratio, control switch means operable to effect over-energization of the load circuit from the generators, relay means responsive a predetermined time after operation of the control switch means to reduce the excitation of the generators to efiect substantially normal energization of the load circuit, and time delay means responsive to the energization of the load circuit operable a predetermined time after operation of the control switch means to effect operation ofthe regulating means.

4. In a power system the combination, of a connected in parallel-circuit relation for supplying current to a work circuit, said generators each having a field winding for controlling the terminal voltage thereof, switch means operable to efi'ect greater than normal excitation of the, field windings, relay means responsive a predetermined interval of time after the opera'tion of the switch means to reduce the excitation of the field windings to substantially normal, means for individually controlling the energization of each field-winding in accordance with the relation between the current supplied by its associated generator and the current supplied by another of the generators, and additional means rendered plurality of low-voltage direct-current generators efi'ective by said relay means for controlling the energization of the field windings to maintain a predetermined division of the current among the generators to control the total current supplied to the work circuit.

5. The combination in a power system, a pair of relatively large direct-current. generators of the commutator type connected in parallel-circuit relation for supplying current to a load circuit,'a field winding for each generator, means for simultaneously varying the energization of the field windings in opposite senses, relay means responsive to the relation between the current supplied by one of the said generators and the current supplied by another'of the generators for controlling the said means and means operable independently of the first mentioned means for collectively controlling the energization of both of the field windings simultaneously in the same sense to control the total current supplied to the load circuit.

6. A power system for supplying current to a load circuit comprising, a plurality of low-voltage direct current generators connected to the load circuit for operation in parallel circuit relation, a field winding for each generator connected to a source of variable voltage, a pilot exciter for each generator having a field winding connected so as to be energized in accordance with the current supplied to the load circuit by its associated generator, an exciter for controlling the energization of each field winding, said exciter having differentially related field excitation windings, means connecting one of said field excitation windings of an exciter to the pilot exciter of its associated generator, and means connecting the other of said field windings, of said exciter to the pilot exciter oi another of the generators.

7. A power system for supplying current to a load circuit comprising, a plurality of relatively low voltage direct-current generators of the commutator type connected in parallel-circuit relation for applying a relatively low voltage to the load circuit, a field winding for each generator, means for effecting energization of each field winding in accordance with the differential between the current supplied by the generator with which said field winding is associated and the total current so as to maintain a predetermined division of the current between the generators, switch means operable to eifect greater than normal energization of said field windings, and time delay means responsive a predetermined interval of time after the operation of the switch means to reduce the energization of the field windings to substantially normal.

8. A power system for supplying electrical energy to a load circuit comprising, a plurality of relatively low voltage direct-current generators of the commutator type connected in parallelcircuit relation to effect the energization oi. the load circuit, a field winding for each generator, means for controlling the energization of each field winding in accordance with the relation between the current supplied by its associated generator and the current supplied by another of said generators, switch means operable for ellecting greater than normal energization of the load circuit, regulating means for maintaining a predetermined fiow of current, relay means controlled by the switch means for effecting a reduction in the energization oi theload circuit after a predetermined interval of time, and time delay means for preventing the operation of the regulating means for a predetermined interval of time after energization of the load circuit.

9. The combination in a power system, of a plurality of direct current generators of the commutator type connected in parallel-circuit relation, a field winding for each generator, a source of energization for the field windings, means for individually controlling the energization of the field windings of each of the generators in accordance with the relation between the current of each generator and the current supplied by all of the generators, switch means operable to effect greater than normal energization of the field windings, relay means for effecting substantially normal energization of the field windings of the generators a predetermined interval of time after the operation of the switch means, regulating means'for independently controlling the energization of the field windings of the generators to control the current, and additional relay means for rendering the regulating means ineffective for a predetermined length of time after operation of the switch means.

10. The combination with resistance welding apparatus having rotatable electrodes for progressively engaging a pipe member which is to be welded, of a plurality of relatively low'voltage direct current generators connected .in parallel circuit relation for supplying current to the elec trodes, means for differentially controlling the output of the generators to maintain a predetermined division of the current among the generators, regulating means operable independently of said means for controlling the output of the generators to maintain a predetermined fiow of current between the electrodes, switch means operable to effect over-excitation of the generators, time delay means for reducing the excitation of the generators to substantially normal a predetermined time after energization, and means responsive to the movement of the pipe member for effecting operation of the regulating means a predetermined period after the initiation or a welding operation, and effecting the energization of the time delay means.

11. The combination with a work circuit having work engaging members for supplying relatively heavy currents to the work, of a plurality of relatively low voltage direct current generators connected in parallel circuit relation for supplying current to the work engaging members, a

field winding for each generator energizable from 0 a source of variable voltage power, switch means energizable to connect the said field winding to the source for greater than normal excitation,-

mined Deriod, control switch means actuated by the work to effect the energization of the said switch and relay means, and means for differentially controlling the energization of the field windings to maintain a predetermined current balance among the generators.

12. The combination in a power system for supplying electrical energy to a load circuit, of a plurality of relatively low voltage direct current commutator type generators connected in parallel circuit relation for supplying a relatively heavy current to the load circuit, a field winding for each generator, a variable voltage source of power for energizing said field windings, means for continuously controlling the voltage of the source after a predetermined period subsequent generators connected in parallel circuit relation I for supplying a relatively large current to the work circuit, a field winding for each of the generators disposed to be initially over-excited, means responsive to the current supplied by each of the generators for difi'erentially varying the excitation of the field windings of the generators to maintain a predetermined current" division therebetween, and control means operable independently of the said means to vary the excitation of the generator field windings so as to maintain a predetermined current flow in the work circuit, relay means operable a predetermined time after energization to reduce the excitation of the field windings, switch means operable a predetermined time after energization to effect the operation of the control means,-and control switch means positioned adjacent the work engaging members actuable by the work to energize the said relay means and switch means.

RALPH H. WRIGHT. 

